Contemporary hard disk drives include an actuator assembly pivoting through an actuator pivot to position one or more read-write heads, embedded in sliders, each over a rotating disk surface. The data stored on the rotating disk surface is typically arranged in concentric tracks. To access the data of a track, a servo controller first positions the read-write head by electrically stimulating the voice coil motor, which couples through the voice coil and an actuator arm to move a head gimbal assembly in lateral positioning the slider close to the track. Once the read-write head is close to the track, the servo controller typically enters an operational mode known herein as track following. It is during track following mode that the read-write head is used to access data written on the track.
Micro-actuators provide a second actuation stage for lateral positioning the read-write head during track following mode. They often use an electrostatic effect and/or a piezoelectric effect to rapidly make fine position changes. They have doubled the bandwidth of servo controllers and are believed essential for high capacity hard disk drives from hereon.
The read-write heads used in contemporary hard disk drives differ from standard idealized models in several ways, most notably in having nonlinearities, which become evident when what is written is compared to what is read on a track in the hard disk drive. Compensating for these nonlinearities improves the channel detection resulting in higher performance and better reliability margin of the hard disk drive, and is often implemented today. The nonlinearity also causes head to media clearance measurement error. This measurement requires very good linear response from the playback device. For hard disk drives using vertical micro-actuation through thermal mechanical or other effect to control the head to media clearance, this nonlinearity tends to cause errors in the head to media clearance and compromise reliability.
The quantitative measure of a read head as a playback device is the transfer curve which plots the relation of the input (in this case, field applied to the reader device) and the output (in this case, the signal from the reader device). A linear playback device yields a transfer curve as a straight line, and a nonlinear playback device yields a transfer curve with high order polynomial components. Currently, the estimation of these nonlinear effects, measuring the transfer curve, is done before the hard disk drive is assembled, often at the level of the slider, the head gimbal assembly, or the head stack assembly. There are several problems and/or disadvantages with this approach. First, special equipment in the form of a device known as a “quasi-static tester” is required to make these estimates. This inevitably increases manufacturing expense, by requiring time on this test stand as well as the potential for tooling and setup costs. Second, the tests are currently performed using a uniform electromagnetic field, which is a far cry from the rapidly varying electromagnetic field induced by flying the read head within a few nanometers of a track on a rotating disk surface. Third, the use of the uniform field also limits the understanding of the distortion caused by the shields of the read head and the soft pole material of the write heads. What is needed is a method allowing the actual non-linearity of the assembled hard disk drive to be calibrated.